The main object of setting a central air-conditioning system is to obtain a comfortable indoor environment, and common central air-conditioning systems mostly adopt circulating water pipelines arranged in a direct return transmission and distribution manner, so that the following several problems always emerge in use: 1, the required temperature can not be always obtained in all tail-end rooms, and particularly after the local tail-end heat load is changed, the temperature can not be timely adjusted to the required temperature; 2, even if a cold or heat production device has enough capacity, a phenomenon of heat unevenness still appears in the system; 3, when the room temperature reaches a required value, the output of a tail end still continuously oscillates; 4, water hammer sound always appears in rooms near a water supply main pipeline; and 5, the rooms have over cold or annoying blowing feeling sometimes. Emergence of these problems not only leads to poor comfortableness, but also leads to increase of the energy consumption of the system. The system with relatively satisfying running condition still consumes more energy by 40%. The reason mainly lies in that the flow required by a tail-end device connected to a pipeline and the water resistance at each position in the pipeline are different and have great influence on the practical flow distribution, so that water flow can not be reasonably distributed to the whole water circulating system, the problem of water flow imbalance exists, and the “overflow” phenomenon of a device close to the water supply main pipeline leads to an “underflow” phenomenon in a device away from the water supply main pipeline. Therefore, in order to improve the comfortableness and achieve the maximum energy-saving effect, a flow balance control must be performed on a circulating water loop.
To compensate the pressure drop in the circulating water pipeline and achieve the effect of balancing system pressure, a water circulating loop adopting a reverse return transmission and distribution manner (there are three transmission and distribution pipes) emerges. The distances that water supply and return main pipes pass by are basically equal due to this manner, namely the consumed on-way resistances are basically identical, so that the resistance of each loop is basically balanced. However, compared with the direct return transmission and distribution manner, the length of the water return main pipe is increased, more time, labor and materials are consumed, part of investment cost is increased and the loss of energy is undoubtedly produced due to the added pipelines in such an arrangement manner. Moreover, because it can not be ensured that all pipe sections have the same pressure drop in the constructed practical engineering, the practical differential pressures are inconsistent.
Aiming at the above-mentioned problems, a method for controlling the circulation of a central air-conditioning water system by using a differential pressure balancing method is proposed, wherein differential pressure balancing valves are additionally arranged based on reverse return transmission and distribution, adjust the opening degrees of the valves by using the differential pressure effect and compensate the resistance change of pipelines by using the pressure drop change of valve elements, so that the differential pressure of the controlled system may be controlled to be constant within a certain flow range, and the differential pressure may be basically unchanged when the working condition is changed. By adopting such a manner, blocking is likely to occur in the differential pressure balancing valves, and each differential pressure balancing valve needs to be debugged, so that the debugging operation of the whole system is relatively complex, and the requirement for experiences of debugging persons is high; and the debugging operation is performed in a static state, and the resistance of each differential pressure balancing valve is relatively fixed after debugging and can not adapt to the change of the load of the system at any time, so that the balance of the system can not be ensured all the time. Moreover, the system is generally debugged with water of 20° C., common water runs normally at 7° C.-12° C. when cold is supplied, and the originally obtained hydraulic balance may be maintained; however, for the water containing 30% of ethylene glycol, the pressure loss of the pipelines under the working condition of 7° C.-12° C. is 4% more than that at 20° C.; and during heating, the average water temperature reaches 60° C., the pressure loss of the pipelines roughly declines by 15%, and the pipeline water resistance is reduced, so that the original balance is broken.
Aiming at the problems of the differential pressure balancing method, flow balance control methods emerge in recent years, and a flow balancing valve is additionally arranged at each tail end of the reverse return transmission and distribution system, so that the flow flowing through the tail end may be directly set according to the design, the flow deviation caused by residual pressure heads and pressure fluctuation of the pipelines is automatically eliminated, the set flow of the tail end may be kept unchangeable no matter how the pressure of the system is changed, the flow adjustment of a pipe network is completed at one time, the debugging operation is changed into simple flow distribution, and thus the hydraulic imbalance problem of the pipe network is effectively solved. The flow of each tail-end unit may be kept consistent through arrangement of the flow balancing valve, so as to solve the original problem that the system is difficult to balance due to many pipelines, inaccurate static balance and the influence of water temperature.
When the flow balancing valve is arranged at the tail end of the system, the set value under the control of the valve is selected according to the design load of the tail-end unit, namely the maximum flow of the flow balancing valve is a passable flow under the condition that the tail-end unit where the flow balancing valve is located is at the maximum design load. The flow balancing valve mainly plays a role in limiting the flow in practical use, and once the required load of the tail-end unit exceeds the original designed maximum load, the flow of a coolant flowing through the tail-end unit is no longer increased, so that the comfortableness declines, and the allocation of energy within a region can not be realized.
Thus it could be seen that selecting an appropriate central air-conditioning device and control method plays an important role in simultaneously meeting the comfortableness and the energy-saving effect.